# Friction acting on a wheel that is initially at rest

1. Jun 15, 2011

### geko11

Hello everyone,

I have a project of mine where I need to find out the force that acts on a wheel to start it rolling and keep it rolling so I can then select the correct motor and gear box.

I have not had to do this kind of physics for years and I am having trouble with this problem:

If a wheel is initially at rest then the standard static friction equation would apply (F = u*Normal) initially to find the force to get the wheel moving and then the rolling friction force equation would be used to work out the force required to keep it going?

or

Because it is a wheel the initial and ongoing friction would be determined by using the rolling static friction equation in both cases?

I am really confused here and would really appreciate the help.

Thanks. :)

2. Jun 15, 2011

### Staff: Mentor

To get the wheel rolling requires static friction (assuming no slipping). But μN gives the maximum amount of friction available to you; the actual friction required depends on the acceleration you are trying to impart. (The actual static friction could be anything from 0 to the maximum value.)

3. Jun 15, 2011

### geko11

ok thanks, so F=uN is max force.

what equation do I use if I have an acceleration in mind to find the force?

4. Jun 15, 2011

### Staff: Mentor

Right.
Newton's 2nd law for rotation and translation.

5. Jun 15, 2011

### geko11

F=m*a......
now I feel like an idiot, but I am glad I asked.

6. Jun 15, 2011

### Staff: Mentor

That will do for translation; for rotation, use Torque = I*alpha.

7. Jun 15, 2011

### rcgldr

8. Jun 15, 2011

### Staff: Mentor

To accelerate the wheel from rest, static friction is required.